1. Field of the Invention
The present invention relates to a distortion compensator and a wireless communication device.
2. Description of the Related Art
An amplifier included in a wireless communication device in the related art tends to generate a distortion component in a carrier when it amplifies the transmission power of the carrier. Such a distortion component may cause a disturbance in the reception frequency band of a cellular phone or cause interference from an adjacent channel such as television or the like. For this reason, it is desirable that a power amplifier achieve low-distortion properties, which do not readily allow a distortion component in the carrier.
In addition, technologies such as HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) have come to be known as viable technologies to be adopted in the WCDMA (Wideband Code Division Multiple Access) in third generation mobile phone systems. These technologies enable higher-speed communication while using carriers in a single bandwidth.
However, the technologies share properties whereby as a code is multiplexed, the crest factor increases. Due to these properties, the power amplifier is known to degrade the carrier with distortion properties even when transmission is executed with uniform mean power. The distortion properties may be improved by, for instance, increasing the current flowing through the amplifier. At the same time, the various circuits constituting the wireless communication device had better achieve better energy efficiency to assure a sufficient length of drive time on a smallest possible battery. Accordingly, the following distortion compensation techniques are attracting a great deal of interest as techniques to be adopted in power amplifiers to improve the distortion properties of the amplifiers without significantly increasing the power consumption.
The distortion compensation techniques, known today as techniques that may be adopted in a transmitter of a wireless communication device, include a negative feedback technique, a predistortion technique and a feed-forward technique.
The negative feedback technique is a method through which a nonlinear distortion is compensated by feeding back an output signal from the power amplifier as a negative feedback to the input side via a negative feedback circuit. Examples of the negative feedback technique include the Cartesian-loop technique, whereby a feedback signal, broken down into an in-phase component and a quadrature component, is fed back as negative feedback.
The predistortion technique is a method through which a pre-distorted signal component is added to an input signal so as to cancel out a distortion that may occur in the power amplifier. By adopting this method, the nonlinear distortion in the power amplifier can be compensated to a certain extent. For instance, Japanese Patent Application Publication No. JP-A-2004-200767 includes a description of the predistortion technique. Unlike the negative feedback technique, the predistortion technique, in which open-loop control is executed, assures superior stability. However, the pre-distorted signal component had better be optimized in conformance to a parameter that determines the properties of the amplifier.
The feedforward technique is a method in which a distortion component occurring in the power amplifier is detected, the detected distortion component is amplified and then the amplified distortion component is subtracted from an output signal of the power amplifier. As in the predistortion technique, open-loop control is executed in the feedforward technique and thus, the feedforward technique, too, assures superior stability. However, a nonlinear distortion component is generated when amplifying the distortion component and the energy efficiency will be compromised in this technique.
Among these nonlinear distortion compensation methods, the predistortion technique, in particular, assures superior stability and superior energy efficiency. For instance, “2005, IEEE MTT-S International Microwave Symposium Session TH2B-6, Third and Fifth order Base-Band Component Injection for Linearization of the Power amplifier in a Cellular phone” and “Japanese Patent Application Publication No. JP-A-2004-200767” quoted here for reference, both disclose a method through which a third-order component or a fifth-order component of a predistortion signal is generated from an orthogonal baseband signal (an I-component and a Q-component) corresponding to a transmission target signal in the digital part.